Muffler assembly with sound absorbing member

ABSTRACT

A muffler is disclosed that includes a housing, an inlet pipe, an outlet pipe, a first partition with first apertures, and a second partition with second apertures. The first partition and housing define a first chamber with which the inlet pipe communicates. The second partition and housing define a second chamber with which the outlet pipe communicates. The second partition is spaced from the first partition to define an intermediate chamber. The muffler also includes a sound absorbing member encapsulated within the intermediate chamber. Exhaust noise entering through the inlet pipe enters the first chamber, moves through the first apertures into the intermediate chamber, moves through the second apertures into the second chamber, and exits through the outlet pipe. Exhaust noise is reduced when passing through the sound absorbing member.

RELATED APPLICATION

This application claims priority from provisional application No. 60/741,114, filed Dec. 1, 2005, which is hereby incorporated herein by reference.

FIELD

The present disclosure relates to a muffler, and more specifically relates to a muffler assembly with sound absorbing member.

BACKGROUND

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

It is known to provide mufflers in exhaust systems to reduce engine exhaust noise. However, conventional mufflers may not reduce exhaust noise sufficiently in some operating conditions. For instance, low frequency exhaust noise can be created in the exhaust system at low engine speeds, and conventional muffler designs may not sufficiently reduce the low frequency sound. Also, standing waves, i.e., resonances within the exhaust system, can be created causing excessive noise. It can be difficult to reduce these standing waves using a muffler because standard reflective muffler tuning techniques are typically not useful for these purposes.

Accordingly, there remains a need for a muffler that more effectively reduces exhaust noise—including low frequency exhaust noise and/or standing waves—within the exhaust system.

SUMMARY

A vehicular exhaust system muffler includes a housing having an input wall and an output wall. First and second apertured partitions inside the housing define an input chamber with one of the input and output walls and an output chamber with another one of the input and output walls. An intermediate chamber is defined by the housing between the first and second partitions. A sound absorbing member is positioned in the intermediate chamber. An inlet pipe enters the housing through the input wall and terminates in the input chamber. An outlet pipe has an end positioned in the output chamber and has another end exiting the housing through the output wall.

Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

FIG. 1 is a sectional plan view of an exhaust system including a muffler;

FIG. 2 is a sectional view of the exhaust system taken along the line 2-2 of FIG. 1;

FIG. 3 is a graph representing exhaust noise reduction using the muffler of FIGS. 1 and 2;

FIG. 4 is a sectional plan view of another embodiment of the exhaust system including a muffler;

FIG. 5 is a graph representing exhaust noise reduction using the muffler of FIG. 4;

FIG. 6 is a schematic view of another embodiment of the exhaust system including a muffler;

FIG. 7 is a schematic view of another embodiment of the exhaust system including a muffler; and

FIG. 8 is a schematic view of another view of the exhaust system including a muffler.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.

Referring to FIGS. 1 and 2, a portion of a vehicle 10 is shown. More specifically, a portion of the exhaust system 12 of the vehicle 10 is shown. The exhaust system 12 includes a muffler 14. It should be appreciated that the exhaust system 12 and the muffler 14 could be included in machines other than a vehicle 10 without departing from the scope of the present disclosure. As will be discussed in greater detail below, exhaust noise within the exhaust system 12 is reduced by the muffler 14.

The muffler 14 includes a housing 16. The housing 16 includes a first wall 18, a second wall 20, and an outer wall 22. The first wall 18, second wall 20, and the outer wall 22 can each be made out of sheet metal. The first and second walls 18, 20 are substantially flat and ovate in shape as shown in FIG. 2, and the outer wall 22 is tubular in shape. One end of the outer wall 22 is coupled to the periphery of the first wall 18. The opposite end of the outer wall 22 is coupled to the periphery of the second wall 20. As such, the first and second walls 18, 20 are substantially parallel and disposed in spaced relation to each other. Also, the housing 16 defines an interior space 24 of the muffler 14 between the first and second walls 18, 20 and the outer wall 22. The first wall 18 includes an inlet aperture 23, and the second wall 20 includes an outlet aperture 25.

The muffler 14 also includes an inlet pipe 26. The inlet pipe 26 is axially straight and can be made out of metal tubing. The inlet pipe 26 includes a first end 28 and a second end 30. The first end 28 extends through the inlet aperture 23. The first end 28 is also in fluid communication with the engine (not shown) of the vehicle 10 and receives exhaust gases therefrom. The second end 30 of the inlet pipe 26 is disposed within the housing 16. As such, exhaust gases are able to flow from the engine of the vehicle 10, through the inlet pipe 26 and into the interior space 24 of the muffler 14 as represented by dashed arrows in FIG. 1.

Furthermore, the muffler 14 includes an outlet pipe 32. The outlet pipe 32 is axially straight and can be made out of metal tubing. The outlet pipe 32 includes a first end 34 and a second end 36. The first end 34 is disposed within the housing 16. The second end 36 extends through the outlet aperture 25. The second end 36 could also be in fluid communication with a tailpipe (not shown) of the vehicle 10. Accordingly, exhaust gases from the engine of the vehicle 10 enters the muffler 14 through the inlet pipe 26 and exits the muffler through the outlet pipe 32 as represented by dashed arrows in FIG. 1.

The muffler 14 further includes a first partition 38. The first partition 38 is substantially flat and ovate in shape as shown in FIG. 2. The first partition 38 can be made out of metal sheet. The periphery of the first partition 38 is coupled to the outer wall 22 so as to create a seal between the first partition 38 and the outer wall 22. In the embodiment shown, the first partition 38 is substantially parallel to both the first wall 18 and the second wall 20. Also, the first partition 38 is disposed in spaced relationship with both the first wall 18 and the second wall 20. In the embodiment shown, the first partition 38 is closer to the second wall 20 than the first wall 18. Accordingly, the first partition 38 and the housing 16 cooperate to define a first chamber 40 between the first partition 38, the second wall 20, and the outer wall 22. The first partition 38 also includes an inlet aperture 42 and an outlet aperture 44. The outlet pipe 32 extends through the outlet aperture 44. The second end 30 of the inlet pipe 26 extends through the inlet aperture 42, and the second end 30 of the inlet pipe 26 terminates within the first chamber 40. Accordingly, the inlet pipe 26 is in fluid communication with the first chamber 40.

In addition, the muffler 14 includes a second partition 46. The second partition 46 is substantially flat and ovate in shape. The second partition 46 can be made out of metal sheet. The periphery of the second partition 46 is coupled to the outer wall 22 so as to create a seal between the second partition 46 and the outer wall 22. In the embodiment shown, the second partition 46 is substantially parallel to the first wall 18, the second wall 20, and the first partition 38. Also, the second partition 46 is disposed in spaced relationship with the first wall 18, the second wall 20, and the first partition 38. More specifically, the second partition 46 is disposed between the first wall 18 and the first partition 38. Accordingly, the second partition 46 and the housing 16 cooperate to define a second chamber 48 between the second partition 46, the first wall 18, and the outer wall 22. The second partition 46 also includes an inlet aperture 50 and an outlet aperture 52. The inlet pipe 26 extends through the inlet aperture 50. The first end 34 of the outlet pipe 32 extends through the outlet aperture 52, and the first end 34 of the outlet pipe 32 terminates within the second chamber 48. Accordingly, the outlet pipe 32 is in fluid communication with the second chamber 48.

As stated, the first and second partitions 38, 46 are disposed in spaced relationship to each other. As such, the first partition 38, the second partition 46, and the housing 16 cooperate to define an intermediate chamber 54 between the first partition 38, the second partition 46, and the outer wall 22.

The first partition 38 includes a plurality of first apertures 56 as shown in FIG. 2. As such, the first chamber 40 is in fluid communication with the intermediate chamber 54. The second partition 46 includes a plurality of second apertures 58. As such, the intermediate chamber 54 is in fluid communication with the second chamber 48. The first and second apertures 56, 58 can be of any suitable size and shape. The first apertures 56 can also be disposed on the first partition 38 in any suitable pattern. Furthermore, the second apertures 58 can be disposed on the second partition 46 in any suitable pattern. In addition, there can be any suitable number of first and second apertures 56, 58.

The muffler 14 also includes a sound absorbing member 60 represented by cross hatching in FIG. 1. The sound absorbing member 60 can be made out of any suitable material. In one embodiment, the sound absorbing member 60 is a collection of a plurality of strands of fiberglass fibers. Suitable fiberglass materials include, but are not limited to ADVANTEX brand fiberglass marketed by Owens Corning and/or POWERTEX brand fiberglass marketed by DBW. The fiberglass material of the sound absorbing member 60 is also commonly referred to the in art as “roving” material. The individual fiberglass fibers of the sound absorbing member 60 are disposed in spaced relationship to each other so as to improve the sound absorbing quality of the sound absorbing member 60.

The sound absorbing member 60 is disposed in and substantially encapsulated within the intermediate chamber 54 as shown in FIG. 1. As such, exhaust gasses entering the muffler 14 through the inlet pipe 26 enter the first chamber 40 and then flow into the intermediate chamber 54 through the first apertures 56 of the first partition 38. The exhaust gasses then flow through the intermediate chamber 54 and through the sound absorbing member 60. The exhaust gasses next flow into the second chamber 48 through the second apertures 58 and into the outlet pipe 32 to be expelled from the vehicle 10.

It should be appreciated that sound created by the engine (exhaust noise) travels through the exhaust system in a substantially similar path. More specifically, exhaust noise enters the muffler 14 through the inlet pipe 26. The sound then enters the first chamber 40 and travels into the intermediate chamber 54 through the first apertures 56 of the first partition 38. The sound then travels through the intermediate chamber 54 and through the sound absorbing member 60. The sound next moves into the second chamber 48 through the second apertures 58 and into the outlet pipe 32 to be expelled from the vehicle 10.

Accordingly, the sound absorbing member 60 is disposed directly within the path of the majority of the exhaust noise. Advantageously, exhaust noise is substantially reduced as it passes through the sound absorbing member 60. The sound absorbing member 60 is especially effective in reducing low frequency sound from the engine. Furthermore, the sound absorbing member 60 reduces standing waves created by the engine.

It should be appreciated that by increasing the density of the sound absorbing member 60 within the intermediate chamber 54, exhaust noise reduction can be further improved. However, increasing the density of the sound absorbing member 60 can also increase back pressure within the exhaust system 12. Accordingly, the density of the sound absorbing member 60 can be adapted to reduce exhaust noise while maintaining an acceptable range of back pressure within the exhaust system 12. In one embodiment, the volume of the intermediate chamber 54 is approximately 5.7 L, and the mass of the sound absorbing member 60 is approximately 500 g, making the density of the sound absorbing member 60 approximately 88 g/L.

In one embodiment, the sound absorbing member 60 is disposed within the intermediate chamber 54 by spraying the fibers of the sound absorbing member 60 directly into the intermediate chamber 54. In another embodiment, the sound absorbing member 60 is first encapsulated within a disposable container, such as a bag made out of polyethylene. The containers are disposed between the first and second partitions 38, 46, and then the first and second partitions 38, 46 and the containers are inserted into the housing 16. During operation, the containers reduce, for instance by burning away, and the sound absorbing member 60 remains within the intermediate chamber 54.

In the embodiment shown, the inlet pipe 26 includes a plurality of transverse apertures 62. The transverse apertures 62 extend through the second end 30 of the inlet pipe 26 so as to be in fluid communication with the intermediate chamber 54. There can be any number of transverse apertures 62 of any suitable size and arranged in any suitable pattern. The outlet pipe 32 also includes a plurality of transverse apertures 64. The transverse apertures 64 extend through the first end 34 of the outlet pipe 32 so as to be in fluid communication with the intermediate chamber 54. As such, exhaust gasses and exhaust noise enters the muffler 14 through the inlet pipe 26 and can move through the transverse apertures 62 of the inlet pipe 26, through the intermediate chamber 54, through the transverse apertures 64 of the outlet pipe 32, and exit the muffler 14 through the outlet pipe 32. As such, some of the exhaust gasses and exhaust noise bypasses the first and second chambers 40, 48 for improved exhaust noise reduction. In one embodiment, most of the exhaust gasses and exhaust noise travels through the first and second chambers 40 as described above, and only a small percentage bypasses the first and second chambers 40, 48. It will be appreciated that the transverse apertures 62 of the inlet pipe 26 and the transverse apertures 64 of the outlet pipe 32 are optional and that the muffler 14 can function without the transverse apertures 62, 64.

Referring now to FIG. 3, a graph 63 is shown which illustrates the performance of one embodiment of the muffler 14. The data of the graph 63 was created by testing the same muffler 14 multiple times. The sound absorbing member 60 had different densities in each test. The graph 63 plots insertion loss, i.e., the exhaust noise reduction or attenuation, on the vertical axis. Exhaust noise frequency is plotted on the horizontal axis. A line 67 represents the performance of the muffler 14 with the sound absorbing member 60 having a mass of approximately 200 grams and a density of 32g/L. A line 69 represents the muffler 14 with the sound absorbing member 60 having a mass of approximately 600 grams and a density of 96 g/L. A line 71 represents the muffler 14 with the sound absorbing member 60 having a mass of approximately 800 grams and a density of 128 g/L. It is appreciated that the volume of the intermediate chamber 54 remains constant because the same muffler 14 was used for each test. Generally, as shown by line 67, the most exhaust noise occurs over the range of frequencies with the muffler 14 having a sound absorbing member 60 of 200 grams. Less exhaust noise occurs with the muffler 14 having a sound absorbing member 60 of 600 grams as shown by line 69. Still less exhaust noise occurs with the muffler 14 having a sound absorbing member 60 of 800 grams as shown by line 71. Thus, FIG. 3 shows that by increasing the density of the sound absorbing member 60, the muffler 14 muffles more exhaust noise. It is also noted that during testing, a standing wave resulted in the exhaust system 12 at approximately 4000 RPM; however, the standing wave was significantly reduced by the muffler 14, especially when the mass of the sound absorbing member 60 was 600 grams or 800 grams. It is understood that the graph 63 represents the performance of just one embodiment of the muffler 14, but the muffler 14 could be adapted to exhibit any desired performance.

Turning now to FIG. 4, another embodiment of the muffler 114 is illustrated, where like numerals increased by 100 are used to indicate like features with respect to the embodiment shown in FIGS. 1 and 2. The muffler 114 is substantially similar to the embodiment of FIGS. 1 and 2, except that the muffler 114 includes a flow-through or by-pass pipe 166. The flow-through pipe 166 can be made out of metal tube stock. The flow-through pipe 166 is disposed entirely within the housing 116 of the muffler 114. The first and second partitions 138, 146 each include flow-through apertures 168 a, 168 b. The flow-through pipe 166 extends through each of the flow-through apertures 168 a, 168 b. Also, the flow-through pipe 166 includes a first terminal end 170 that is in fluid communication with the first chamber 140 and a second terminal end 172 that is in fluid communication with the second chamber 148. Accordingly, exhaust gas and exhaust noise is able to pass through the flow-through or by-pass pipe 166 without passing through the sound absorbing member 160.

As shown in FIG. 4, the muffler 114 also includes a valve 174. The valve 174 is operatively connected to the second terminal end 172 of the flow-through pipe 166 so as to selectively limit flow of exhaust gasses and exhaust noise through the flow-through pipe 166. In one embodiment, for instance, the valve 174 includes a flap 176 that is pivotally attached to the flow-through pipe 166. The valve 174 also includes a biasing member 178, such as a torsion spring. The biasing member 178 is retained by a pin 180 that extends through the biasing member 178 and that is attached at each end to flanges 182 a, 182 b of the flow-through pipe 166. The flap 176 is sized so as to substantially cover the opening in the second terminal end 172 of the flow-through pipe 166. Also, the biasing member 178 biases the flap 176 toward the opening in the second terminal end 172 to cover the opening of the flow-through pipe 166.

Accordingly, exhaust gasses travel from the inlet pipe 126 into the first chamber 140 as described above. When pressure in the first chamber 140 is relatively low, the biasing member 178 biases the flap 176 such that the flap 176 remains in position covering the opening in the second terminal end 172 of the flow-through pipe 166. Thus, substantially all of the exhaust gas and exhaust noise passes through the sound absorbing member 160 and out of the muffler 114 through the outlet pipe 132 as described above. However, when pressure in the first chamber 140 is relatively high, the flap 176 pivots against the force of the biasing member 178 to allow exhaust gas and exhaust noise to pass through the flow-through pipe 166 from the first chamber 140 to the second chamber 148 and bypass the sound absorbing member 160.

In one embodiment, the pressure in the first chamber 140 remains low enough to keep the flap 176 closed when the vehicle engine is operating at relatively low RPMs. As such, low frequencies and/or standing waves that result from low engine RPMs are reduced by the sound absorbing member 160. The pressure in the first chamber 140 increases enough to open the flap 176 when the vehicle operates at higher RPMs. It will be appreciated that the biasing member 178 can be adapted to cause the valve 174 to open at any desired engine exhaust flow rate. As such, back pressure within the exhaust system 112 can be reduced for improved engine performance at higher RPMs. It will also be appreciated that the exhaust noise will be less noticeable at these high RPMs because of other external noise, such as wind noise and the like.

Referring to FIG. 5, a graph 184 is shown that represents the performance of the muffler 114 of FIG. 4. The graph 184 plots “tail pipe sound pressure level” (TPSPL), i.e., the exhaust noise level, on the vertical axis. Engine speed is plotted on the horizontal axis. A first line 186 represents a desired maximum exhaust noise level. A second line 188 represents the performance of the muffler 114 when the valve 174 is pinned shut and unable to open. A third line 190 represents the performance of the muffler 114 when the valve 174 opens and closes normally. As shown by the second line 188, exhaust noise remains generally lower than the maximum exhaust noise level across the entire range of engine speeds. However, it is understood that because the valve 174 remains closed, back pressure in the exhaust system 112 could detrimentally affect engine performance. As represented by line 190, the valve 174 opens at approximately 3500 RPM in the embodiment shown. Before the valve 174 opens, i.e., when the engine speed is below approximately 3500 RPM, the exhaust noise is below the desired maximum. Then, when the valve 174 opens, i.e., when the engine speed is above approximately 3500 RPM, the exhaust noise exceeds the maximum exhaust noise level. However, as explained above, the valve opens 174 to reduce back pressure in the exhaust system 112 and improve engine performance. Also, the increased exhaust noise occurring when the valve 174 is open is more acceptable because external noises, e.g., wind noises, mask the exhaust noise. It is understood that the graph 184 represents just one embodiment of the muffler 114, but the muffler 114 could be adapted to exhibit any desired performance.

Referring now to FIG. 6, another embodiment of the exhaust system 212 is shown, where like numerals increased by 200 indicate like features with respect to the embodiment shown in FIGS. 1 and 2. As shown, the muffler 214 is coupled to the inlet pipe 226 and the outlet pipe 232. More specifically, the housing 216 of the muffler 214 is disposed between the inlet pipe 226 and the outlet pipe 232. The first and second partitions 238, 246 divide the muffler 214 into a first chamber 240, a second chamber 248, and an intermediate chamber 254 disposed between the first and second chambers 240, 248. The first partition 238 includes a plurality of first apertures 256, and the second partition 246 includes a plurality of second apertures 258. The inlet pipe 226 terminates in the first chamber 240 so as to be in fluid communication therewith. The outlet pipe 232 terminates in the second chamber 248 so as to be in fluid communication therewith. The sound absorbing member 260 is encapsulated in the intermediate chamber 254.

Accordingly, exhaust gases and exhaust noise pass from the inlet pipe 226, into the first chamber 240, through the first apertures 256, into the intermediate chamber 254, through the sound absorbing member 260, through the second apertures 258, into the second chamber 248, and out through the outlet pipe 232. It is appreciated that exhaust noise is reduced as it passes through the sound absorbing member 260. It is also appreciated that the cross sectional area of the housing 216 of the muffler 214 is substantially larger than the cross sectional area of the inlet pipe 226 and the outlet pipe 232. This reduces the amount of back pressure created within the exhaust system 212 during operation.

Referring now to FIG. 7, another embodiment of the exhaust system 312 is shown, where like numerals increased by 300 are used to indicate like features with respect to the embodiment shown in FIGS. 1 and 2. The exhaust system 312 is substantially similar to the embodiment shown in FIG. 7, except that the muffler 314 includes a flow-through or by-pass tube 366 similar to the embodiment shown in FIG. 5. The flow-through tube 366 extends through the first partition 338 and the second partition 346 so as to be in fluid communication with both the first and second chambers 340, 348. Also, the valve 374 is disposed within the second chamber 348. Accordingly, during operation, the valve 374 can remain closed at lower engine speeds, and exhaust gasses and exhaust noise pass substantially through the sound absorbing member 360. However, when pressure in the first chamber 340 is high enough, the pressure will force the valve 374 open to allow some exhaust gas and exhaust noise to move through the flow-through tube 366 to thereby reduce back pressure within the exhaust system 312.

Referring now to FIG. 8, another embodiment of the exhaust system 412 is shown, where like numerals increased by 400 are used to indicate like features with respect to the embodiment shown in FIGS. 1 and 2. As shown, the muffler 414 includes the first partition 438 and the second partition 446. The first partition 438 includes a plurality of first apertures 456, and the second partition 446 includes a plurality of second apertures 458. The intermediate chamber 454 is defined between the first and second partitions 438, 446. The sound absorbing member 460 is disposed within the intermediate chamber 454.

The muffler 414 also includes a third partition 491 that is disposed within the housing 416 such that an absorbing chamber 492 is defined between the third partition 491, the first wall 418, and the outer wall 422. A second sound absorbing member 461 is disposed within the absorbing chamber 492.

The muffler 414 also includes a fourth partition 493 that divides the first chamber 440 into an input chamber 494 and a return chamber 495. Furthermore, the muffler 414 includes a return pipe 496 that extends through the fourth partition 493 such that the input chamber 494 and the return chamber 495 are in fluid communication.

The inlet pipe 426 extends through the outer wall 422 and into the input chamber 494. The inlet pipe 426 terminates in the input chamber 494. The outlet pipe 432 extends through the first wall 418, through the third partition 491, through the first partition 438, and through the second partition 446. The outlet pipe 432 terminates within the second chamber 448. Also, the outlet pipe 432 includes a plurality of transverse apertures 464 such that the outlet pipe 432 and the absorbing chamber 492 are in fluid communication. It should be appreciated that the absorbing chamber 492 is completely enclosed but for the transverse apertures 464.

During operation, exhaust gas and exhaust noise flow through the muffler 416 as represented by dashed arrows in FIG. 8. More specifically, the exhaust gas and noise enter the muffler 416 through the inlet pipe 426 into the input chamber 494. A portion of the gas and noise flow through the first apertures 456 and into the intermediate chamber 454. The other portion of the gas and noise flow through the return pipe 496 to the return chamber 495, and then through the first apertures 456 into the intermediate chamber 454. As discussed above, the exhaust noise is reduced as it passes through the sound absorbing member 460. The gas and noise then pass through the second apertures 458 into the second chamber 448 and into the outlet pipe 432. Substantially all of the exhaust gas is then expelled out of the muffler 414 through the outlet pipe 432. However, some of the exhaust noise passes out of the muffler 414 through the outlet pipe 432, and some of the noise passes through the transverse apertures 464 into the absorbing chamber 492 for further reduction by the second sound absorbing member 461. Accordingly, exhaust noise can be significantly reduced because of the effects of both the sound absorbing members 460, 461.

In summary, the muffler 14, 114, 214, 314, 414 disclosed above reduces exhaust noise in an effective manner by causing the majority of exhaust noise to move through the sound absorbing member 60, 160, 260, 360, 460, 461. This can reduce undesirable low frequency exhaust noise and/or standing waves within the exhaust system 12, 112, 212, 312, 412. 

1. A vehicular exhaust system muffler comprising: a housing having an input wall and an output wall; first and second apertured partitions inside the housing defining an input chamber with one of the input and output walls, defining an output chamber with another one of the input and output walls, and an intermediate chamber defined between the first and second partitions; a sound absorbing member positioned in the intermediate chamber; an inlet pipe entering the housing through the input wall and terminating in the input chamber; and an outlet pipe having an end positioned in the output chamber and another end exiting the housing through the output wall.
 2. The muffler of claim 1 wherein the input chamber is adjacent the output wall and the inlet pipe extends from the input wall through the output chamber and the intermediate chamber before terminating in the input chamber, the inlet pipe further including apertures therein enabling fluid communication between the inlet pipe and the intermediate chamber.
 3. The muffler of claim 2 wherein the output chamber is adjacent the input wall and the outlet pipe extends from the output chamber through the intermediate and input chambers to the output wall, the outlet pipe further including apertures therein enabling fluid communication between the intermediate chamber and the outlet pipe.
 4. The muffler of claim 1 further comprising a flow-through pipe extending from the input chamber through the intermediate chamber to the output chamber and having a valve at one end thereof operative to vary an open area of the one end as a function of exhaust flow pressure.
 5. The muffler of claim 1 wherein the sound absorbing member comprises roving material.
 6. The muffler of claim 5 wherein the roving material comprises strands of fiberglass fibers.
 7. The muffler of claim 5 wherein the roving material comprises strands of basalt wool.
 8. The muffler of claim 1 further comprising a third solid partition defining with the housing a supplemental sound absorbing chamber containing a supplemental sound absorbing member, and wherein the outlet pipe extends through the supplemental sound absorbing chamber and the intermediate chamber, the outlet pipe having perforations enabling fluid communication between the outlet pipe and the supplemental sound absorbing chamber.
 9. The muffler of claim 8 wherein the sound absorbing member comprises a roving material having a first density and the supplemental sound absorbing member comprises a roving material having a second density.
 10. The muffler of claim 9 wherein the first density is less than the second density.
 11. The muffler of claim 1 wherein the input chamber is adjacent the input wall and the inlet pipe extends through the input wall and terminates in the input chamber.
 12. The muffler of claim 11 wherein the output chamber is adjacent the output wall.
 13. The muffler of claim 12 further comprising a flow-through pipe extending from the input chamber to the output chamber and having a valve at one end thereof operative to vary an open area of the one end as a function of exhaust pressure. 